1. Field of the Invention
This invention relates to an injection sleeve for die casting which is employed for die-casting a molten metal such as aluminum (Al), an aluminum alloy and zinc (Zn).
2. Background of the Invention
The die casting method wherein a molten metal such as aluminum, an aluminum alloy or zinc is supplied to an injection sleeve of a die casting machine, the molten metal is injected into a die casting mold by a plunger tip at a high speed, a high pressure is applied on the molten metal and the molten metal is solidified in the mold, is widely employed in the production of metal parts in the industrial fields making automobiles, electric machines and the like. The reason is that the time required for casting a single piece of product is significantly short and the production cost is inexpensive compared with the other casting methods.
In the die casting method, the casting cycle wherein the molten metal is transmitted into the inner portion of the mold of the casting machine at a high flow rate and is solidified, is finished in a very short time period compared with those of the other casting methods. This casting cycle is repeated at a high frequency. Therefore, a portion contacting the molten metal, especially the injection sleeve or the die casting mold requires a material provided with an excellent corrosion resistance and erosion resistance against the molten metal and the thermal shock resistance against the repetitively performed heating and cooling cycle.
In casting aluminum or an aluminum alloy having a comparatively high casting temperature, these parts are required to stand an extremely severe condition. Conventionally, a material has been utilized for these parts, wherein an alloy steel, such as SKD 61, is heat-treated and further treated with a nitriding-treatment on its surface.
Further, there is a considerable difference between conditions of contacting the molten metal between the injection sleeve and the die casting mold. When the temperature of the molten metal which is transmitted into the sleeve is lowered, the viscosity of the molten metal increases and further a portion thereof is precipitated, the structure of the cast product becomes inhomogeneous which adversely influences the product property. Therefore, the cooling of the molten metal should be avoided as much as possible at the injection sleeve. On the other hand, the mold is sufficiently cooled down since the inner portion thereof is a portion for solidifying the molten metal.
The corrosion is significant on the inner face of the injection sleeve, especially in the region beneath a feeding port, since this region contacts the molten metal having a relatively high temperature. The service life of the injection sleeve made of an alloy steel is short even if the nitrization-treatment is performed thereon. However, no promising material has been found as the material for the conventional injection sleeve other than the alloy steel. The injection sleeve of an alloy steel, such as SKD 61, is currently employed while cooling the portion thereof wherein the corrosion is significantly caused, in a range allowable for the injection sleeve.
Further, a plunger tip (a piston-like member padded with Colmoloy alloy on its surface and the inside thereof is water-cooled) should rapidly be moved in the injection sleeve and the molten metal should be pressurized in the mold in a short period of time. Therefore, a sufficient lubricity should be maintained between the inner face of the injection sleeve and the plunger tip. In this view, the injection sleeve made of an alloy steel is problematic, and a lubricant wherein carbon is mixed with water should be supplied by about several tens ml per a casting. This lubricant is thermally decomposed into gases by the heat of the molten metal. These gases are incorporated in the inner portion of the molten metal which causes pores remained in the cast product.
In recent times, trials have been performed wherein ceramics having an essentially high corrosion resistance against the molten metal and the thermal shock resistance due to its small thermal expansion, for instance, sialon in Japanese Unexamined Patent Publication No. 72464/1988, or silicon nitride in Japanese Examined Patent Publication No. 2949/1985, is employed in the injection sleeve for die casting.
However, the toughness and the thermal shock resistance of these ceramic sleeves are not sufficiently high. Even when these sleeves are reinforced by fitting the ceramic sleeve into a metal cylinder, almost no reinforcement effect can be provided in the temperature range of usage, since the thermal expansion (3 to 4.times.10.sup.-6 /.degree. C.) is significantly smaller than the thermal expansion of the metal cylinder (in case of SKD 61, 13 to 14.times.10.sup.-6 /.degree. C.). Therefore, the condition of usage is limited and the sleeve is destructed by mechanical impact or by thermal shock in its usage, which does not reach the stage of practical use.
A cermet sintered body was proposed in Soviet Powder metallurgy and Metal Ceramics No. 8(44) p665-670, 1966 and in Japanese Unexamined Patent Publication No. 196353/1987, which was provided with a hard ceramic phase of a Mo-Ni complex boride and a metal matrix phase of an Ni base alloy. Further, the chemical corrosion resistance and the oxidation resistance of a cermet sintered body can be promoted by further adding chromium to the cermet sintered body.
The present inventors proposed a cermet sintered body composed of a hard ceramic phase whose major component is a Ni-Mo complex boride, a Ni-W complex boride or a Ni-Mo-W complex boride and a metal matrix phase whose major component was an Ni-Mo alloy, in Japanese unexamined Patent Publication No. 143236/1988, and showed that the cermet sintered body was a material having a large strength and hardness at an elevated temperature, compared with those in a WC-Co cemented carbide and the like. In Japanese Unexamined Patent Publications No. 143236/1988, the inventors proposed a material wherein a carbide was added to the cermet sintered body of this species to promote the strength and the hardness at an elevated temperature.
Further, the inventors proposed a mold (cooled) for die-casting the molten metal such as aluminum or zinc, as a specific use of these cermet sintered body in Japanese Unexamined Patent Publication No. 299740/1990.
However, concerning the problem whether the cermet sintered body of this system can be used as the injection sleeve for aluminum which is employed under a severer condition, the adaptability thereof has been unclarified in case wherein the cooling is not performed, since the cermet sintered body incorporates the metal matrix phase whose major component is nickel and the metal matrix phase is comparatively weak at corrosion by the molten aluminum and the like, and, therefore, the adaptability of the cermet sintered body without cooling has been doubtful.